The only way to know for sure about drilling and tapping is to ask em by email or phone call, email and toll free number is in the FAQ and Help sticky. H&R's FAQ says they won't drill and tap their shotgun barrels for a scope rail, tho. FWIW, someone here had his 45/410 barrel drilled and tapped by his gunsmith, so it can be done, but by H&R, I dunno.
Tim